The present invention relates to water treatment methods and devices and, more particularly, to a water treatment method and water treatment device suitable for use for sterilizing microbes in drinking water, a circulatory bath, or the like.
It is previously known that intense ultrasonic waves have a sterilizing effect and sterilizing devices for sterilizing bacteria that cause problems in drinking water or circulatory baths or the like are being developed using the sterilizing effect of such ultrasonic waves. Examples of water treatment devices constituting sterilizing devices of this type are described in Laid-open Japanese Patent Publication No. 2001-9448 and Laid-open Japanese Patent Publication No. 2001-334264. In previous water treatment devices as described above, cavitation is generated by applying ultrasonic waves of large amplitude (about 1 μm) utilizing the resonance of an ultrasonic wave generating mechanism and bacteria are destroyed mechanically by means of the high temperature, high pressure, and high-speed water currents generated on collapse thereof. Although it is commonly stated that sterilization using ultrasonic waves is achieved by oxidation produced by an acousto-chemical effect, if frequencies lower than about 28 KHz are employed, it has been confirmed that bacteria larger than about 5 μm are mechanically disintegrated.
Further, technology relating to ultrasonic wave devices in which the frequency of the ultrasonic waves can be changed so as to maintain the resonance point of an ultrasonic wave resonator that generates ultrasonic waves that fluctuates depending on the condition of the liquid is disclosed in Laid-open Japanese Patent Publication No. H. 6-71226, Laid-open Japanese Patent Publication No. H. 7-265794, and Laid-open Japanese Patent Publication No. 2001-212514.
When water treatment devices described above are actually employed, they are subject to the problem that the frequency of the ultrasonic waves that is optimum for generation of cavitation fluctuates depending on the effect of parameters such as water temperature, flow rate and impurities, so that it is difficult for optimal cavitation to be generated. In addition, if the devices described above are employed in a condition in which water is absent, the amplitude becomes abnormally large, giving rise to the problem that the device is in danger of being destroyed and the further problem that the device itself may be exfoliated by the cavitation generated by the device, causing changes in the resonant frequency.
FIG. 1 shows an example of an ultrasonic device in which sound waves are generated by producing vibration by applying high frequency to a magnetostriction or piezoelectric element have been employed for a long time in the industrial field. As shown in FIG. 1, their basic construction is that a vibrator 3 is introduced into a liquid 2 contained in an enclosure 1 and high frequency from an oscillator 7 is amplified by an amplifier 6. The amplified high frequency signal is applied to a vibration source 4 comprising a magnetostriction or piezoelectric element. This high frequency vibration is transmitted and magnified by passing through a horn 5 and directed into the liquid 2 in the form of a sound wave.
If the acoustic pressure from the vibrator is raised by increasing the power applied to the vibrator 2, the liquid 2 is subjected to a rapid change of pressure, causing the static pressure of the liquid to drop below the vapor pressure, with the result that cavities are generated by air dissolved in the liquid or by vaporization of the liquid. This phenomenon is called cavitation. When the bubbles produced by cavitation collapse, a shock of 1000 to 5000 atmospheres is generated in a short period in a minute region; this has an extremely large destructive power. This action is called cavitational erosion and is being taken up as a research topic in many places, as shown by Okada et al., Machinery Research (Kikai no Kenkyu) Vol. 49, No. 11 (1997) pp 1187 to 1196.
Such cavitation has been employed for a long time in emulsifying devices and ultrasonic washing devices in the industrial field. In FIG. 1, an enclosure 1 is filled with liquid 2 and a plurality of vibrators 3 are arranged therein. The high frequency waves from an oscillator 7 are amplified by an amplifier 6 and applied to a vibration source 4 comprising a magnetostriction or piezoelectric element; the resulting vibration is directed into the liquid 2 from the tip of horn 5.
As disclosed in Kato, Cavitation (New edition) Maki Shoten at p. 84, the number of cavitation bubble nuclei described above is inversely proportional to the diameter of the bubble nuclei. With this device, the acoustic pressure that was applied and the mechanism of bubble generation were not elucidated. Moreover, there were the following problems.                (1) The region where cavitation bubbles were generated was small and it was difficult to produce cavitation in the entire body of liquid.        (2) If cavitation was produced in the entire body of liquid, a large liquid flow rate could not be achieved.        (3) The size of the cavitation bubbles was difficult to control.        (4) Erosion of the tip of the horn occurred due to cavitational erosion, producing particles which became mixed with the liquid.        
There have been instances of deaths caused by pneumonia due to Legionella bacteria proliferating in water at hot springs and the social problem has also risen of diarrhea epidemics due to cryptosporidium bacteria present in mains water supplies. It is desirable to provide a device for destroying and sterilizing pathogenic bacteria, in particular, Legionella bacteria, cryptosporidium bacteria or Escherichia coli bacteria while maintaining the quality of hot spring water or mains water.
In 24-hour hot-water baths and the like for domestic use, pathogenic bacteria proliferate, and there are reported cases of newborn babies or old persons who have low resistance to bacteria being infected. Previously known methods of sterilization/disinfection of pathogenic bacteria in water include chlorine sterilization, ultraviolet ray sterilization, ozone sterilization and disinfection using membrane filtration. However, in the case of a circulatory hot-water bath, there are respectively the following problems.                (1) Chlorine: this causes a change of the pH value and produces a change of the water quality. It gives an unpleasant feeling to the bather, since a smell of chlorine is generated. The chlorine is consumed simply by persons taking a bath so that no residual chlorine is left; managing the amount of chlorine with each bath is therefore a troublesome task. It is considered that high chlorine concentrations affect the skin. The chlorine is neutralized in 1 to 2 minutes in hot alkaline water of pH greater than 7, falling to a concentration of 0.1 ppm.        (2) Ultraviolet rays: the effectiveness of these is reduced by the fact that they cannot pass through cloudy water. The conditions of use are restricted to colorless transparent hot water, practically equivalent to mains tap water in which the chromaticity of the hot water is less than 5 degrees and which is of turbidity less than 2 degrees.        (3) Ozone: the quality of the hot water is affected by decolorization and deodorization. Furthermore, since high ozone concentrations are toxic to the human body, de-ozonization treatment using activated carbon or the like is required.        (4) Membrane filtration: this is undesirable as a filter for a bath, since bathing agents and the like are removed. Also, blockage of the pores occurs.        
Prior examples are found in Laid-open Japanese Patent Application No. H. 2001-9448 and Laid-open Japanese Patent Application No. H. 2001-334264.